High voltage resistant members for outdoor service are known, which typically are made of a fiberglass reinforced resin bonded rod surrounded by a suitable weather resistant material of a shape and construction which will shed rain. Metal fixtures are attached to the rod, for example in service as an insulator, to allow connection to a conductor at one end and to a support structure at the other end.
A number of problems have been found in design, manufacture and application of high voltage resistant members. The problems in part are related to the nature of the weather resistant material surrounding the rod. Epoxy resins were among the earliest materials used as the weather resistant material and it was found that employment of hydrated alumina in large quantities improved their tracking and erosion resistance. This improvement, due to the hydrated alumina, also occurs with other polymers. The epoxy resin formulations which provide the best electrical performance, however, generally are rigid and subject to cracking at low temperatures, especially if the rod is loaded mechanically in tension or cantilever bending.
Various elastomers have been used to improve flexibility of the weather resistant material. These elastomers have included ethylene-propylene-diene monomer rubber (EPDM), ethylene-propylene monomer rubber (EPM), butyl rubber, silicone resins, fluorocarbon polymers and the like. EPM and EPDM are particularly attractive from a cost viewpoint. But the elastomers generally are formed by molding under pressure, though a few are castable. The castable elastomers usually suffer from inadequacies in this regard, such as poor tear strength, and usually are not able to incorporate sufficiently large quantities of hydrated alumina to give the needed performance in tracking and erosion resistance. The moldable elastomers require prohibitively large molds and presses, if the high voltage members are to be molded in one piece.
A number of patents relating generally to construction and manufacture of such high voltage resistant members are listed hereafter:
U.S. Pat. Nos. 1,991,700, Rost; 2,683,185, Morrison; 2,732,423, Morrison; 2,945,912, Imhof; 3,001,004, Black; 3,001,005, Soinnenberg; 3,118,968, Moussou; 3,152,392, Coppack et al.; 3,328,515, Vose; 3,291,899, Ward et al.; 3,356,791, McGowan; 3,358,076, Rebosio; 3,446,741, Hervig et al.; 3,531,580, Foster; 3,544,707, Gamble; 3,549,791, Yonkers; 3,626,083, Minter et al.; 3,800,111, Holmstrom; 3,898,372, Kalb; 4,217,466, Kuhl; 4,246,696, Bauer et al.
British Pat. Nos. 816,926, Coppack; 902,197, Bannerman; 915,052, Sweetland; 1,066,209, Rebosio; 1,116,197, Rebosio; 1,182,045 Rebosio; 1,226,265, British Insulated; 1,292,276, Clabburn et al.
West German No. 28 32 543, Trevisan et al.; 1,189,600 Leeds.
U.S. Pat. No. 3,898,372 discloses a method which circumvents the molding problem of EPM by molding each shed separately and then mounting the sheds over a fiberglass rod and filling the space between the sheds and rod with a silicone grease. This is a simple expedient allowing individually molded sheds to be produced at low cost. However, such a construction creates numerous potential access points for water invasion from the outside to the rod. When the rod becomes wet, it fails electrically. Such wetting could occur during high pressure water washing of the member, a practice used by many electric utilities to remove accumulated contamination.
British Pat. No. 1,182,045 seeks to eliminate the numerous joints by use of a preformed elastomeric sleeve. The internal surface of the sleeve is treated in order to render it bondable to the rod and its inside diameter must be sufficiently greater than the outside diameter of the rod to permit adhesive to be introduced in order to bond the rod and the sleeve together. Adhesive voids can result from such a cumbersome procedure.
U.S. Pat. No. 3,112,357 discloses a conductor with hardenable resinous compositions, but which are not particularly resistant to tracking and erosion when exposed to weather and contamination. Consequently a porcelain housing has been required for outdoor use. Such a procelain housing is both heavy and fragile.
U.S. Pat. No. 4,217,466 discloses a composite insulator comprising a rod of nonsaponifiable resin reinforced with glass fibers of low alkali content, an intermediate layer of a nonsaponifiable moisture repellant polymer surrounding the rod, and screens surrounding said intermediate layer, the screens also being of a moisture repellant nonsaponifiable polymer and containing a filler. Suitable polymers for the screens are those containing ether or acetal groups, for example, silicone rubber or ethylene-propylene rubber. A preferred polymer for the intermediate layer is a polyfunctional polyorganodimethylsiloxane. The screens may be prefabricated and pushed on a prefabricated rod containing the intermediate layer, or may be cast molded onto said prefabricated rod.
U.S. Pat. No. 4,246,696 discloses a composite insulator comprising a prefabricated glass fiber rod surface treated with a silane, a strengthened extruded rubber layer, and a plurality of prefabricated screens slipped over and bonded to the rubber layer. The strengthening of the extruded layer is carried out by incorporating pyrogenic-obtained silicic acid in the rubber.
West German Offenlegungsschrift No. 28 32 543 discloses an electric insulator for medium and high voltage comprising a glass fiber stem and a skirted coating of elastic organic material, said material being compatible with the stem, for example, an ethylene-propylene elastomer with favorable antitracking and antierosion characteristics, high elasticity and aging and flame resistance. Preferably, the skirted coating is a unitary element of the assembly, but my comprise a tubular sleeve and individual shirts. The bond between stem and skirted coating is made with a curing mixture of weakly unsaturated olefinic polymers, which polymers are analogous to and compatible with the ethylene-propylene coating, and which cure at room temperature.